This invention relates generally to a safety glass window construction which is generally suitable for heating vehicle windows including the windshield, side and rear windows. More particularly, it relates to a glass sandwich having an electrically conducting transparent panel for the inner layer which is substantially free from glare when light is transmitted through the safety glass window. Specifically, it relates to an electrically-conducting transparent panel for a safety glass window construction having particular bus-bar electrode means as part of a thermoplastic inner layer along with a protruding array of resistance wires which enables simplified and more reliable assembly of the final window construction along with improved heating of the window.
In a pending patent application entitled, ELECTRICALLY HEATED WINDOW, Ser. No. 166,064, filed July 26, 1971 in the names of George A. Gruss, George J. Polanka, and Leslie H. Pfeiler, which is assigned to the assignee of the present invention, there is disclosed and claimed a composite sheet member which comprises a layer of transparent thermoplastic polymer that has been heat-shrunk from its original dimensions by heating to an elevated temperature having partially embedded in one major surface a plurality of wrinkled resistance wires which lie in closely spaced relationship and are oriented with respect to one another in non-parallel random fashion so that portions of said individual resistance wires protrude from said major surface of the thermoplastic polymer layer and form a three-dimensional heating wire construction with improved fabrication and operational advantages. As one embodiment of said novel transparent heating panel which forms the otherwise conventional inner layer for a laminated safety glass window construction, there is also described in said earlier filed application certain bus-bar electrode means consisting of a single strip of electrically conducting material secured at each end of the individual resistance wires to form a parallel-connected electrical circuit therebetween. The single strip electrodes are in direct physical contact with protruding portions of the individual resistance wires and provide a generally reliable termination which is not subject to mechanical rupture of the heating wires at the electrical terminations. On the other hand, these single bus-bar strips are secured by heat-bonding directly to one major surface of the thermoplastic polymer sheet with the individual resistance wires being secured to the opposing side of the strips. Such termination can result in occasional failure of direct electrical contact between the strips and the heating wires which is caused by excessive flow of the polymer material when the final window assembly is made. Even without excessive flow of the plastic material, there can be increased contact resistance from chemical reaction between the copper bus-bars and the plastic material after the final assembly has been put into use.
In said earlier filed application referenced above, there are also disclosed methods and apparatus for assembling the electrically heated transparent panel as well as the final safety glass window assembly. In one method of fabricating the electrically heated transparent panel, as well as the final safety glass window assembly. In one method of fabricating the electrically heated transparent panel, the electrical resistance wire forming the heating wire pattern is prestressed to provide wrinkles when the resistance wire is not under tension. The prestressed wire is thereafter applied under tension to one major surface of the transparent thermoplastic polymer sheet in a pattern having closely spaced parallel orientation with respect to the individual wires whereupon the tension on the individual resistance wires is relaxed while retaining physical contact between the pattern of resistance wires and the surface of the thermoplastic polymer sheet. During the subsequent heating step, the polymer sheet as well as the wires can expand and contract along the direction of the wire path. The composite member is then heated by applying electrical power to the individual resistance wires so that portions of the wires which are in physical contact with the surface of the polymer sheet become embedded and the remaining portions of the individual wires protrude from the major surface of the plastic sheet. The composite member having a bus-bar electrode configuration and the resistance wire pattern as above described can thereafter be assembled as the inner layer of an otherwise conventional safety glass window construction in a variety of already known methods. The final heat-bonding step which produces the glass laminate is also generally carried out under sufficient heating and compressional forces that significant flow of the polymer material occurs.
A particular apparatus to fabricate the composite member is also described in said copending application which utilizes a collapsible drum member upon which the member is formed. In so doing, the thermoplastic polymer sheet is first wrapped upon the drum followed by application of separate heating and bus-bar electrodes, and then followed by winding of prestressed resistance wire in the form of a helical coil over the wound polymer sheet. The composite member is then formed by collapsing the drum a predetermined decrease in diameter and thereafter thermally tacking or heat-bonding the resistance wire and the subjacent single element bus-bar electrodes to the polymer sheet.
The products and methods of the present invention constitute improvements over said earlier disclosure which can utilize the same general apparatus to assemble the electrically heated transparent panel of the present invention. Consequently, a detailed description of the same apparatus need not be repeated in the present specification except to explain the method of carrying out the present invention.